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A value given to a breakpoint applies to all the other breakpoints wider than it (unless overridden, as you can read later in this page). For example, xs={12} sizes a component to occupy the whole viewport width regardless of its size.
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To control space between children, use the spacing propprops. The spacing value can be any positive number, including decimals and any string. The prop is converted into a CSS property using the theme.spacing() helper.